The need and capability of weighing railcars in motion has been well established as disclosed for example in prior U.S. Pat. Nos. 3,155,184; 3,734,216; and 4,416,342 incorporated herein by reference. Disclosed in my prior U.S. patent is a method and apparatus to effect such weighing by obtaining strain gauge inflection measurements along a controlled length of unsupported track. Operation of the system thereof is dependent on discreet electrical signals being generated by individual inflections for each axle of the passing railroad car. The signals combine to yield a constant sum of the bending moments which when summed for all four axles can be converted, from a bridged output signal, to the actual weight of the railcar.
The foregoing system disclosed in my '342 patent has performed well and met with commercial success in providing substantial accuracy of result within about one-half of 1 percent compared to individual cars weighed uncoupled at rest on a platform scale. This inaccuracy however marginal, is generally attributed to various factors operable in combination and previously unrecognized. One factor believed to be contributing is the unpredictable compression of the wood fibers contained in the wood crossties of the track forming the fulcrum nearest the lead and trailing ends of the unsupported track section. That is, the prior system utilized otherwise standard railroad beds for the approaches including the wood crossties. Those crossties disposed at the transition immediately before and after the adjacent unsupported section of track tend to carry a disproportionate larger load than do the upstream and downstream crossties having helper ties on both sides by which the received load can be distributed. It is believed that the weight of the passing train wheels cause the wood fibers of the adjacent ties to temporarily yield during the time period in which inflection measurements are being taken thereby distorting their measurement accuracy. Another equally significant factor is wheel eccentricity resulting from either manufacturing tolerance and/or in-service wear. The eccentricity can typically vary from 0.005 inches to 0.031 inches, and in the course of motion can generate an additional weighing force of approximately 1/10 of 1 percent of the actual weight of the wheel being weighed. With eight wheels on a typical ordinary freight car, the error produced by such eccentricities can in effect compound the inaccuracy to on the order of about 1/10 of 1 percent greater or lesser than the actual weight of the car.
Yet further factors contributing to the aforementioned inaccuracies are believed to result from previously unrecognized effects of temperature on the rail modulus of elasticity as well as calibration drift in the instrumentation utilized for receiving the bridged output signal from the strain gauges.